Method for purifying hydrogen based gas mixture using a lithium- exchanged X zeolite

ABSTRACT

Process for the separation of the hydrogen contained in a gas mixture contaminated by carbon monoxide and containing at least one other impurity chosen from the group consisting of carbon dioxide and saturated or unsaturated, linear, branched or cyclic C 1  -C 8  hydrocarbons, comprising bringing the gas mixture to be purified into contact, in an adsorption region, with at least: 
     one first adsorbent selective at least for carbon dioxide and for C 1  -C. hydrocarbons and 
     one second adsorbent which is a zeolite of faujasite type exchanged to at least 80% with lithium, the Si/Al ratio of which is less than 1.5, in order to remove at least carbon monoxide (CO).

FIELD OF THE INVENTION

The invention relates to a process for the purification ofhydrogen-based gas mixtures contaminated by various impurities,including carbon monoxide and at least one other impurity chosen fromcarbon dioxide and saturated or unsaturated, linear, branched or cyclicC₁ -C₈ hydrocarbons.

The process of the invention makes it possible in particular to improvethe conventional processes for the separation of hydrogen of PSA type,or pressure-swing adsorption processes, using zeolites as adsorbent.

BACKGROUND OF THE INVENTION

The production of high-purity hydrogen is of great interestindustrially, the latter being widely used in many synthetic processes,such as hydrocracking, the production of methanol, the production ofoxoalcohols and isomerization processes.

In the prior art, PSA processes have proved to be very efficient in theseparation of gas mixtures and in particular in the production of purehydrogen or oxygen from gas mixtures contaminated by various impurities.PSA processes take advantage of the adsorption selectivity of a givenadsorbent for one or a number of the contaminating substances of the gasmixture to be purified.

The choice of the adsorbent is problematic: it depends, on the one hand,on the nature of the mixture to be treated. As a general rule, theadsorbents are selected according to their ability to adsorb and todesorb a specific compound. In fact, PSA processes involve the operationof pressure cycles. In a first phase, the adsorbent bed separates atleast one constituent of the mixture by adsorption of this constituenton the adsorbent bed. In a second phase, the adsorbent is regenerated bylowering the pressure. At each new cycle, it is therefore essential forthe desorption to be efficient and complete, so that there is anidentical regenerated state at each new cycle. However, it is clear thatthis ability to adsorb and then desorb a specific constituent of a gasmixture is a function of the specific operating conditions of the PSAprocess envisaged and in particular of the temperature and pressureconditions.

A distinction must therefore be made between PSA processes intended forthe production of oxygen, which generally operate at adsorptionpressures of less than 5×10⁵ Pa, and PSA processes intended for theproduction of hydrogen, which can involve adsorption pressures ofbetween 5×10⁵ and 70×10⁵ Pa.

However, insofar as the mixture to be purified generally comprises morethan one impurity, it is desirable for the adsorbent to be able toadsorb and then desorb not one alone but a number of these impurities.

In point of fact, the adsorption profile and selectivity for a givenconstituent are often influenced by the presence, in the gas mixture, ofother impurities, this being due, for example, to possible competitionor to poisoning of the adsorbent.

These various considerations account for the complexity of the problemof the optimization of PSA processes by improvement of the adsorbent.

Recent studies have shown that, in the case of mixtures containingnitrogen, oxygen, hydrogen, methane and argon or helium,lithium-exchanged zeolites make possible a marked improvement in theperformance characteristics. The result in particular of the variousresearch studies carried out is that the selection criteria to be takeninto account in choosing the adsorbent are its nitrogen adsorptioncapacity, its nitrogen/oxygen selectivity, its mechanical strength (thepacking of the adsorbent having to be possible over a certain height,without crushing) and the pressure drop occasioned, this naturally beingthe situation in the case of gas mixtures comprising both nitrogen andoxygen for the purpose of purification of the oxygen.

Reference will be made, for example, to documents U.S. Pat. No.5,152,813 and U.S. Pat. No. 5,258,058 and to Patent ApplicationEP-A-0,297,542, which describe the use of lithium-exchanged zeolites oftype X in PSA processes intended for the production of oxygen.

The teaching of these documents, however, is not generally applicable tothe purification of gas mixtures containing impurities of the carbonmonoxide, carbon dioxide or C₁ -C₈ hydrocarbon type, the presence ofwhich modifies the profile for adsorption of nitrogen by the zeolite. Inpoint of fact, these impurities are the most frequently encountered inPSA units for the purification of hydrogen.

Moreover, the adsorption pressures involved in the prior art cited,generally being well below 5×10⁵ Pa, do not correspond to thosegenerally used for PSA processes for the production of hydrogen.

Indeed, as regards the production of hydrogen from a hydrogen-based gasmixture containing CO, CO₂, CH₄, NH₃, H₂ S, N₂ and H₂ O as impurities,document U.S. Pat No. 3,430,418 provides the combination of two types ofadsorbent, the first, which is an active charcoal, removing CH₄, CO₂ andH₂ O and the second, which is a zeolite of type A containing calcium,making possible the removal of the nitrogen and carbon monoxide. Untilnow, so as to improve the performance characteristics of PSA processesfor the production of hydrogen, and in particular with a view toobtaining a better hydrogen yield, the number and the arrangement of theadsorbent beds operating in parallel has essentially been varied.Documents U.S. Pat. No. 4,381,189 and FR-A-2,330,433 illustrate inparticular such an approach.

SUMMARY OF THE INVENTION

The present invention is based on the discovery that the combination ofa specific zeolite with at least one second adsorbent of silica gel,active charcoal or prefilter charcoal type makes possible the removal ofimpurities of the carbon monoxide, carbon dioxide, saturated orunsaturated, linear, branched or cyclic C₁ -C₈ hydrocarbon and nitrogentype from a gas mixture containing hydrogen, while resulting in asignificant increase in the productivity. As used here, the termproductivity denotes the ratio of the volume of hydrogen produced,measured under standard temperature and pressure conditions, per hourand per volume or weight of adsorbent.

More specifically, the invention relates to a process for the separationof the hydrogen contained in a gas mixture contaminated by carbonmonoxide and containing at least one other impurity chosen from thegroup consisting of carbon dioxide and saturated or unsaturated, linear,branched or cyclic C₁ -C₈ hydrocarbons, which comprises bringing the gasmixture to be purified into contact, in an adsorption region, with atleast:

one first adsorbent selective for at least carbon dioxide and C₁ -C₈hydrocarbons,

and one second adsorbent which is a zeolite of the faujasite typeexchanged to at least 80% with lithium and the Si/Al ratio of which isless than 1.5, in order to remove at least carbon monoxide (CO).

The process of the invention is more particularly appropriate for theremoval of carbon monoxide from hydrogen-based gas mixtures containing,in addition, other impurities such as carbon dioxide or saturated orunsaturated, linear, branched or cyclic C₁ -C₈ hydrocarbons of themethane, ethane, butane, propane, benzene, toluene or xylene type.Likewise, the gaseous nitrogen optionally present in the gas mixture isseparated from the hydrogen by adsorption on the specific adsorbent bedemployed in the process of the invention. Advantageously, at least part,and preferentially most, of the nitrogen optionally present in the gasmixture to be purified is removed by adsorption on a third adsorbent bedplaced or interposed, that is to say "sandwiched", between the bed ofthe first adsorbent selective for at least carbon dioxide and C₁ -C₈hydrocarbons and the bed of the second adsorbent intended to removemainly CO. The adsorbent constituting the third bed is preferably chosenfrom zeolites, such as zeolite 5A.

According to a preferred embodiment, the gas mixture contains carbonmonoxide, carbon dioxide, methane, nitrogen and hydrogen.

Mention may be made, as gas mixtures which may be suitable, of the gasmixtures resulting from catalytic cracking units, thermal crackingunits, catalytic reforming units or hydrotreating units.

The purity of the hydrogen resulting from the process of the inventionis at least 99.999% when the gas mixture to be purified comprises morethan 45% of gaseous hydrogen. However, this purity can reach up to99.999999% or more, depending on the operating conditions involved andthe amount of adsorbent used. The purification of a gas mixturecontaining less than 45% of gaseous hydrogen is undesirable, insofar asit would require an excessive amount of adsorbent and a disproportionatesize of the plants in order to be able to achieve an acceptable purity.It goes without saying that the greater the proportion of hydrogen in astarting gas mixture, the greater will be the purity of the hydrogenrecovered at the outlet of the adsorption region. The process of theinvention is considered as giving the best results when the percentageof hydrogen in the gas mixture to be treated is at least 70%.

As a general rule, in the context of the invention, the adsorptionregion is maintained at a pressure of between 5×10⁵ and 70×10⁵ Pa duringthe operation of bringing the gas mixture to be purified into contactwith the first and second adsorbents. However, a higher pressure doesnot adversely affect the management of the purification. However, with aconcern to save energy and because of the high cost ofpressure-resistant plants, pressures above 70×10⁵ Pa are generallyavoided. Pressures of less than 5×10⁵ Pa are not usually employed forthe production of hydrogen by adsorption of impurities on an adsorbentbed, for reasons of efficiency. The pressure prevailing in theadsorption region will preferably be maintained at a value of less than50×10⁵ Pa, better still less than 30×10⁵ Pa. The adsorption region islikewise preferably maintained above 5×10⁵ Pa, preferentially above15×10⁵ Pa.

The temperature of the incoming gas mixture and of the adsorption regionis not determining and is generally kept constant during the phase ofadsorption of the impurities. This temperature is usually between 0 and50° C., preferably between 30 and 45° C., during the adsorption.

The first and second adsorbents are arranged in the adsorption region sothat the gas mixture passes through them one after the other. It hasbeen found that the efficiency of the separation could be optimized byplacing, at the inlet of the adsorption region, the adsorbent selectiveat least for carbon dioxide and for C₁ -C₈ hydrocarbons and, at theoutlet of the adsorption region, the adsorbent of faujasite typecontaining lithium intended to remove at least CO.

This result can be explained by the fact that the efficiency ofadsorption of the zeolite of faujasite type is increased once theimpurities of C₁ -C₈ hydrocarbon and carbon dioxide type have beenhalted by the first adsorbent.

Use may be made, as adsorbent selective at least for carbon dioxide andfor C₁ -C₈ hydrocarbons, of an active charcoal, a prefilter charcoal, asilica gel or a mixture of these various adsorbents in any proportion.When such a mixture is chosen, it is preferable to arrange the variousconstituents of the mixture, in the adsorption region, in the form ofseparate layers so that the gas mixture comes into contact with eachlayer in turn.

The silica gels which can be used according to the invention are thosecommonly used in the art. These gels are commercially available, inparticular from Solvay (Sorbead gel). The prefilter charcoals are activecharcoals of high porosity and low relative density. The activecharcoals and prefilter charcoals are, for example, sold by Norit,Carbotech, Ceca, Pica or Chemviron.

The second adsorbent is advantageously a zeolite of faujasite typeexchanged to at least 80% with lithium.

Zeolites are a group of hydrated natural or synthetic metallicaluminosilicates, the majority of which exhibit a crystalline structure.Zeolites differ from one another in their chemical composition, theircrystalline structure and their physical properties. Zeolite crystalsare schematically composed of interlinked SiO₄ and AlO₄ tetrahedranetworks. A certain number of cations, for example cations of alkalimetals and alkaline-earth metals, such as sodium, potassium, calcium andmagnesium, enclosed within the crystal lattice, ensure the electricalneutrality of the zeolite.

The zeolites of faujasite type, also denoted in the art by zeolite X,are crystalline zeolites of formula:

    (0.9±0.2)M.sub.2/n O: Al.sub.2 O.sub.3 :2.5±0.5 SiO.sub.2 :yH.sub.2 O

in which M represents an alkali metal or alkaline-earth metal, n is thevalency of the metal M and y takes any value between 0 and 8, dependingon the nature of M and the degree of hydration of the zeolite. DocumentU.S. Pat. No. 2,882,244 relates to this specific type of zeolite.

Zeolites X in which the Si/Al ratio is less than 1.5 are selectedaccording to the invention. This ratio is preferably between 1 and 1.2,it being understood that a value of 1 is more particularly recommended.

Zeolites X are commercially available, in particular from the followingcompanies: Byer, UOP, Ceca, Ueticon, Grace Davison or Tosoh. Zeolites13X provided by these distributors are, in particular, appropriate asstarting materials for the preparation of lithium-exchanged zeolites Xwhich can be used according to the invention as adsorbent.

That said, the process of the invention is not limited to the use ofcommercial faujasites. The use of a zeolite with a higher or lowerporosity than that of industrial zeolites X which are currentlycommercially available is not, for example, ruled out.

According to the invention, the zeolites can be in the form ofcrystalline powders or of agglomerates. Zeolite agglomerates areobtained conventionally by making use of standard agglomerationprocesses. The agglomerated zeolite can, for example, be prepared bymixing a crystalline zeolite powder with water and a binder (generallyin the powder form) and then spraying this mixture onto zeoliteagglomerates acting as agglomeration seed. During the spraying, thezeolite agglomerates are continuously rotated about themselves. This canbe achieved by placing the agglomerates in a reactor rotating aboutitself around a rotational axis, the rotational axis preferably beinginclined with respect to the vertical direction. By this process,commonly denoted in the art by "snowball" process, agglomerates in theform of balls are obtained. The agglomerates thus obtained are thensubjected to firing at a temperature of between approximately 500 and700° C., preferably at a temperature in the region of 600° C. The personskilled in the art can resort, as example of binder, to a clay, such askaolin, silica or alumina. The agglomerated zeolite thus obtained, whichcomprises a binder, can be used in the preparation of binder-freeagglomerated zeolite, which can also be used in the process of theinvention. It is possible, so as to convert the binder into the zeolitephase, in fact to subsequently fire zeolite agglomerates containingbinder, whereby, after crystallization, binder-free zeolite agglomeratesare obtained.

According to the invention, the zeolites X which can be used asadsorbent are subjected to a subsequent treatment aimed at introducinglithium cations into the crystal lattice. This is achieved by ionexchange, a portion of the M⁺ cations initially contained in the zeolitebeing exchanged with lithium cations.

The combination of the first and second adsorbents described aboveresults in an improvement in the purification of the gas mixturecontaining hydrogen and in the overall productivity when the secondadsorbent is a zeolite of faujasite X type exchanged to at least 80%with lithium.

Zeolite exchanged to at least 80% with lithium is understood to mean azeolite in which at least 80% of the AlO₂ ⁻ units are associated withlithium cations.

Any known process of the state of the art which makes it possible to endup with a zeolite of faujasite type exchanged to at least 80% withlithium can be employed.

Zeolites of faujasite type exchanged to more than 90% with lithium aremore particularly preferred.

Before the zeolites containing lithium are used, they must be activated.According to the invention, activation of a zeolite is understood tomean its dehydration, that is to say the removal of the water ofhydration contained in the zeolite. As a general rule, it is seen tothat the partial pressure of water in the gas in contact with thezeolite is less than approximately 4×10⁴ Pa, preferably 1×10⁴ Pa, afteractivation. Processes for the activation of zeolites are known in theart. One of these methods consists in subjecting the zeolite to apressure of approximately 1×10⁴ Pa to 1×10⁶ Pa while passing a stream ofan inert gas through the bed of adsorbent composed of the zeolite andwhile heating the zeolite to a temperature of between 300 to 650° C. ata rate of temperature rise of approximately 0.1 to 40° C. per minute. Asan alternative, the zeolite can be activated by maintaining it under avacuum of approximately 1×10⁴ Pa or less while heating the zeolite to atemperature of approximately 300 to 650° C. without having to resort topurging with an inert gas. Another alternative consists in activatingthe zeolite by a process using microwaves, as described in Document U.S.Pat. No. 4,322,394.

It is possible, in making use of the adsorbent bed, a priori to combinethe first and second adsorbents in any ratio by weight. Nevertheless, itwas possible to find that a ratio by weight of the first adsorbentselective at least for carbon dioxide and for C₁ -C₈ hydrocarbons to thesecond adsorbent of zeolite type of between 10/90 and 85/15 isparticularly advantageous from the viewpoint of the efficiency of thepurification and of the overall productivity. It was possible to observeempirically that this ratio is ideally between 50/50 and 80/20,preferably between 60/40 and 80/20.

It is known, so as to produce hydrogen continuously, to arrange inparallel a number of adsorbent beds which are alternately subjected to acycle of adsorption with adiabatic compression and of desorption withdecompression.

Such plants are employed in particular in PSA pressure-swing adsorptionprocesses. The treatment cycle to which each adsorbent bed is subjectedcomprises the stages consisting in:

a) passing a hydrogen-based gas mixture contaminated by carbon monoxideand containing at least one other impurity chosen from the groupconsisting of carbon dioxide and C₁ -C₈ hydrocarbons into an adsorptionregion comprising at least:

one first adsorbent bed composed of a first adsorbent selective at leastfor carbon dioxide and for C₁ -C₈ hydrocarbons and:

one second adsorbent bed composed of a second adsorbent which is azeolite of faujasite type exchanged to at least 80% with lithium, theSi/Al ratio of which is less than 1.5, in order to remove at least CO;

b) desorbing the carbon monoxide and the other impurity or impuritiesadsorbed on the first and second adsorbents by setting up a pressuregradient and progressively lowering the pressure in the adsorptionregion, so as to recover the carbon monoxide and the other impurity orimpurities via the inlet of the adsorption region; and

c) raising the pressure in the adsorption region by introduction of astream of pure hydrogen via the outlet of the adsorption region.

Thus, each adsorbent bed is subjected to a treatment cycle comprising afirst phase of production of hydrogen, a second phase of decompressionand a third phase of recompression.

It is clear that, by adjusting the operating conditions of stage a) inaccordance with the preferred embodiments described above, the result isan improvement in the yield and productivity as well as an improvementin the purity of the hydrogen recovered at the outlet. Thus, thepurification of a gas mixture containing more than 70% of hydrogen andcomprising nitrogen, methane, CO and CO₂ as impurities will preferablybe opted for, which mixture will be brought into contact with anadsorbent bed composed of active charcoal and of faujasite exchanged tomore than 90% with lithium, the ratio by weight of the active charcoalto the faujasite preferably being between 50/50 and 80/20.

The adsorption region is preferably maintained at a temperature ofbetween 0 and 80° C.

The capacity of the adsorbent bed is limited by the maximum size whichcan be used, either because of the mechanical strength of the individualadsorbent particles or because of the maximum size which can be used forshipping the containers containing the adsorbents. For this reason, theoperation of 4 to 10 adsorbent beds arranged in parallel is standard inthe art.

In order to optimize PSA processes, the phases of decompression and ofcompression of the various adsorbent beds are synchronized: it is inparticular advantageous to introduce stages of pressure equalizationbetween two adsorbent beds, one of these two beds being in thedecompression phase and the other in the recompression phase.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is now described with reference to implementationalExamples 1 and 2 and to the appended figures.

FIG. 1 represents diagrammatically a plant for the implementation of aPSA process for the production of hydrogen comprising ten adsorbent beds1 to 10.

FIG. 2 represents the change in the pressure within an adsorption regionduring a treatment cycle for the purification of a hydrogen-based gasmixture by a PSA process.

FIG. 3 represents the variations in the adsorption capacity of variouszeolites as a function of the adsorption pressure.

FIG. 4 represents variations in the adsorption capacity of alithium-exchanged zeolite of type X according to the invention (curveLiX) and of a conventional zeolite 5A (curve 5A) as a function of theadsorption pressure.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1, only the pipes through which hydrogen circulates, at a givenmoment, have been represented. More specifically, at the moment underconsideration, the beds 1 to 3 are in the production phase, the beds 4to 7 are in the decompression phase and the beds 8 to 10 are in therecompression phase.

The beds 1 to 3 are fed with the gas mixture to be purified via thepipes 11, 12 and 13 respectively. The pipes 11, 12 and 13 are eachconnected to a source of gas mixture 14 via one and the same pipe 15into which the pipes 11, 12 and 13 run. The purified hydrogen isrecovered at the outlet of the adsorption regions 1 to 3 via the pipes16, 17 and 18.

All three of the pipes 16 to 18 run into a pipe 19 which conveys thepurified hydrogen, coming from the pipes 16, 17 and 18, to a storagechamber 21 via the pipe 19. A portion of the hydrogen produced iswithdrawn from the pipe 19 via the pipe 22 and conveyed to the adsorbentbed 10, which is then at the end of the recompression phase: thepressure is thus equalized between the adsorbent beds 1 to 3 in theproduction phase and the adsorbent bed 10 which is at the end of thetreatment cycle.

During this same period, pressure equalization of the adsorbent beds 4and 8, on the one hand, and 4 and 9, on the other hand, is carried out.To do this, the respective inlets of the adsorption regions 4, 8 and 9are hermetically sealed. The adsorbent beds 4 and 8 are placed incommunication with one another via a pipe 23. The adsorbent bed 4,entering into the decompression phase, is then under a relatively highhydrogen pressure, whereas the adsorbent bed 8, which is at thebeginning of the recompression phase, is at a much lower pressure. As aresult of the pressure difference existing between the adsorbent beds 4and 8, hydrogen is driven from the adsorbent bed 4 to the adsorbent bed8, which contributes to the recompression of the adsorbent bed 8 and tothe concomitant decompression of the adsorbent bed 4. The pipe 23 runsmore specifically into pipes 24 and 25, the pipe 24 being connected tothe adsorbent bed 8 and the pipe 25 being connected to the adsorbent bed9. Thus, via the pipes 23 and 25, the adsorbent beds 4 and 9 are alsoplaced in communication with one another: in fact, pressure equalizationof the adsorbent beds 4 and 8, on the one hand, and 4 and 9, on theother hand, is carried out simultaneously.

In the same way, pressure equalization between the adsorbent beds 5 and7 is carried out, these two beds being placed in communication with oneanother via a pipe 26. Here again, insofar as the pressure prevailingwithin the adsorbent bed 5 is greater than the pressure prevailingwithin the adsorption region 7, the hydrogen flows from the adsorbentbed 5 to the adsorbent bed 7. This thus results in pressureequalization. Nevertheless, insofar as the adsorbent beds 5 and 7 are,in one case, in the course of decompression and, in the other case, atthe end of the decompression phase, it is desirable not to equalize thepressures of these adsorption regions but, on the contrary, to lower thepressure prevailing in the adsorption region 7 with respect to thepressure prevailing in the adsorption region 5. This is achieved byallowing the excess hydrogen to discharge from the adsorption region 7via the inlet 27 of the adsorption region 7.

The adsorption bed 6 is also in the decompression phase. Its pressure islowered simply by discharging the hydrogen via the inlet 28 of theadsorption region 6. It is at this decompression stage that thedesorption of the impurities adsorbed on the adsorbent bed takes place.

EXAMPLES

In the following examples, two gas mixtures M1 and M2 are purified, thecompositions of these gas mixtures being indicated in Table 1 below.

In this table, the percentages are percentages by volume.

                  TABLE 1    ______________________________________    CH.sub.4 (%)                CO (%)   CO.sub.2 (%)                                   N.sub.2 (%)                                          H.sub.2 (%)    ______________________________________    M1    3         3        22      2      70    M2    3         3        22      0      72    ______________________________________

To do this, a plant analogous to that described with reference to FIG.1, comprising 10 adsorbent beds, is used.

The treatment cycle employed is shown diagrammatically in FIG. 2. Morespecifically, the change in the pressure within an adsorbent bed overthe course of time has been represented in FIG. 2.

In all cases, the adsorbent beds are packed with active charcoal, on theone hand, and with a zeolite, on the other hand.

The active charcoal used is of the type of those generally employed inthe various processes for the separation of hydrogen by pressure-swingadsorption (PSA/H₂).

In the case of Comparative Examples 1, 3 and 4, the zeolite is zeolite5A sold by the company Procatalyse under the reference 5APS.

In the case of Comparative Example 2, the zeolite is zeolite 5A sold bythe company layer under the reference Baylith K.

In the case of Examples 1 to 3 in accordance with the invention, thezeolite used is a zeolite X exchanged to 90% with lithium.

The zeolites Baylith K® (Bayer) and 5APS® (Procatalyse) are zeolites Acontaining, as exchangeable cations, Na⁺ and Ca²⁺ ions and exhibitingpores with a size of approximately 5 Å.

The zeolite 5APS®, which is in the form of extrusion products with adiameter of 1.6 mm and 3.2 mm, is additionally characterized by:

a loss on ignition of between 1 and 1.5% at 550° C.;

a bulk density of between 0.69 and 0.73 g/cm³ ;

a specific heat of 0.23 kcal.kg⁻¹. ° C⁻¹ ;

a heat of adsorption of water of at most 1000 kcal/kg;

a static adsorption of water of between 17.0 and 19 g of water per 100 gof adsorbent at a relative humidity of 10%, and

a static adsorption of water of between 20 and 22 g of water per 100 gof adsorbent at a relative humidity of 60%.

The zeolite X exchanged to 90% with lithium is prepared in the followingway, starting with a faujasite 13X exhibiting an Si/Al ratio of 1.25 andcontaining approximately 20% of binder:

A 1.94N aqueous lithium chloride solution containing 60 mmol/l of sodiumchloride, the pH of which has been adjusted beforehand to 8 by additionof lithium hydroxide, is percolated through a column packed with 1 kg ofthe faujasite. During this operation, the column is maintained at atemperature of 95° C.

The lithium-exchanged zeolite obtained is characterized by theisothermal curve passing through the points marked □ in FIG. 3. Thisisothermal curve was plotted at 20° C. by volumetric analysis by meansof a Sorptomatic MS 190 apparatus from Fisons, after activation of thezeolite at 400° C. under vacuum for 8 hours.

More specifically, this curve represents the variations in the nitrogenadsorption capacity, expressed in cm³ per gram, as a function of theadsorption pressure (expressed in bars).

Before they are placed in the various adsorption regions, the zeolitesare activated under vacuum at 400° C. for 8 hours.

The active charcoal is placed at the inlet of the adsorption region, thelithium-exchanged zeolite of faujasite type being placed at the outletof the adsorption region, so that these two adsorbents form two distinctsuperimposed layers.

The temperature of the adsorbent beds is maintained at 40° C.

A number of experiments are then carried out in order to test theefficiency of the process of the invention; in these experiments, thevalue of the adsorption pressure (achieved at the end of the compressionphase) and the value of the desorption pressure (achieved at the end ofthe decompression phase) are modified and the resulting yield andresulting productivity are determined.

The productivity P is defined here as the ratio of the volume ofhydrogen produced, measured under standard temperature and pressureconditions, per hour and per m³ of adsorbent.

The yield Y of the process corresponds to the ratio of the volume ofpure hydrogen produced, measured under standard temperature and pressureconditions, to the volume of hydrogen contained in the effluent gas tobe purified, also measured under standard temperature and pressureconditions.

In the examples below, the yields and productivities reported in Tables2 and 3 are relative yields and productivities.

In fact, Comparative Example 1 was chosen as reference, that is to saythat, for this example, the relative yield and relative productivitywere set at 100: this example illustrates more specifically thepurification of the mixture M1 described in Table 1 in the presence ofan adsorbent composed of 70% by weight of active charcoal and of 30% byweight of zeolite 5APS, the desorption pressure being set at 2×10⁵ Paand the adsorption pressure being 20×10⁵ Pa.

Consequently, in the case of all the other examples, the relative yieldsand productivities Y_(r) and P_(r) are given respectively by theequations: ##EQU1## where Y and P are as defined above and Yc₁ and Pc₁are the true yield and true productivity respectively determined in thecase of Comparative Example 1.

The results obtained in the case of the comparative examples have beenreported in Table 2 below as a function of the pressure conditionsselected and of the active charcoal/zeolite ratios by mass used:

                  TABLE 2    ______________________________________                 Active    Com-         charcoal/                          Gas   Adsorp-                                      Desorp-    Rela-    para-        zeolite  mixture                                tion  tion  Rela-                                                 tive    tive         ratio by to be pressure                                      pressure                                            tive produc-    Ex.  Zeolite mass     purified                                (bars)                                      (bars)                                            yield                                                 tivity    ______________________________________    1    5APS    70/30    M1    20    2     100  100    2    Baylith 70/30    M1    20    2     96.1 98         K    3    5APS    75/25    M2    23    1.6   100  100    4    5APS    75/25    M2    33    1.6   100  100    ______________________________________

The results obtained by using the lithium-exchanged zeolite inaccordance with the process of the invention have been collected inTable 3 in the case of the following three examples:

                  TABLE 3    ______________________________________         Active char-                   Gas mix- Adsorp-                                  Desorp-         coal/zeolite                   ture to  tion  tion        Relative         containing Li                   be puri- pressure                                  pressure                                        Relative                                              produc-    Ex.  ratio by mass                   fied     (bars)                                  (bars)                                        yield tivity    ______________________________________    1    70/30     M1       20    2     103.5 113.4    2    75/25     M2       23    1.6   101.7 112.3    3    75/25     M2       33    1.6   101.6 111.3    ______________________________________

In the case of Examples 1 to 3, the purity of the hydrogen produced was99.999%.

It clearly results from these results that the combination of activecharcoal and of lithium-exchanged zeolite X leads to better values ofthe yield and the productivity.

The isothermal curves of variation in the nitrogen adsorption capacityin the case of each of the zeolites studied in Examples 1 to 3 andComparative Examples 1 to 4, as a function of the adsorption pressure,are represented in the appended FIG. 3.

In these curves, the amount Q of nitrogen adsorbed (expressed in cm³ pergram) has been given on the ordinates and the adsorption pressure(expressed in bars) has been given on the abscissae. The points relatingto the lithium-exchanged zeolite of Examples 1 to 3 are marked □; thepoints relating to the zeolite A Baylith K® (Bayer) are marked ∘ and thepoints relating to the zeolite A 5PAS® (Procatalyse) are marked Δ.

These curves were plotted at 20° C. by volumetric analysis by means of aSorptomatic MS 190 apparatus from Fisons, after activation of thezeolites at 400° C. under vacuum for 8 hours.

It clearly results from these curves that the adsorption capacity of thezeolite containing lithium is greater. Likewise, the adsorption capacityof the zeolite Bylith K, sold by Bayer, is greater than that of thezeolite 5APS.

In point of fact, in the light of the results obtained above, the yieldand productivity obtained in PSA processes for the production ofhydrogen are the best for the lithium-exchange zeolite X and the worstin the case of the zeolite A Baylith K sold by Bayer.

It is thus demonstrated that the adsorption capacity of a zeolite fornitrogen, until now regarded as an important criterion in choosing themost efficient zeolite, does not have a direct relationship with theyield and productivity which are finally obtained in PSA processes forthe production of hydrogen.

The isothermal curves of variation in the carbon monoxide (CO)adsorption capacity of a zeolite of faujasite type exchanged to 87% withlithium according to the invention (curve LiX) and of a zeolite 5A(curve 5A) are represented in FIG. 4.

In these curves, the amount Q of carbon monoxide (CO) adsorbed(expressed in Scm³ /g) has been given on the ordinates and theadsorption pressure (expressed in bars) has been given on the abscissae;these measurements were carried out at 30° C.

These isothermal curves clearly show that the lithium-exchanged zeoliteX (LiX) has, for a given pressure, a much higher adsorption capacity forcarbon monoxide than the conventional zeolite of type 5A.

Likewise, it is found that the CO respiration of a lithium-exchangedzeolite X according to the invention is markedly superior to that of aconventional zeolite of type 5A. Indeed, the respiration of a zeolite isdefined as the difference between the adsorption capacity for a pure gasby this zeolite at the high partial pressure, or adsorption pressure,and the adsorption capacity for the gas by the zeolite at the lowpressure, or desorption pressure.

Consequently, for an adsorption pressure of 23 bar, a desorptionpressure of 1.6 bar and a gas having a CO content of approximately 3%, amean CO partial pressure in the adsorption phase (on zeolite) ofapproximately 0.69 bar is obtained and a pressure of approximately 0.24bar was evaluated in the desorption phase.

Consequently, for a conventional zeolite of type 5A, the amount of COadsorbed in the adsorption phase is approximately 18.3 Scm³ /g andapproximately 11.1 Scm³ /g in the desorption phase, which corresponds toa respiration of approximately 7.2 Scm³ /g.

Analogously, for a lithium-exchanged zeolite X in accordance with theinvention, the amount of CO adsorbed in the adsorption phase isapproximately 35.9 Scm³ /g and approximately 25.2 Scm³ /g in thedesorption phase, which corresponds to a respiration of approximately10.7 Scm³ /g.

It thus immediately appears that to use a lithium-exchanged zeolite inplace of a conventional zeolite of type 5A makes it possible to obtain,surprisingly, a respiration in the case of carbon monoxide (CO) which isimproved by approximately 48%.

The industrial and commercial advantage of the process of the presentinvention is thus convincingly demonstrated.

We claim:
 1. Process for the separation of hydrogen contained in a gasmixture contaminated by carbon monoxide and containing at least oneother impurity selected from the group consisting of carbon dioxide andsaturated or unsaturated linear, branched or cyclic C₁ -C₈ hydrocarbons,which comprises bringing the gas mixture to be purified into contact, inan adsorption region, with at least:one first adsorbent selective atleast for carbon dioxide and for C₁ -C₈ hydrocarbons, and one secondadsorbent which is a faujasite zeolite exchanged to at least 80% withlithium, and having a Si/Al ratio which is less than 1.5, in order toremove carbon monoxide.
 2. Process according to claim 1, wherein the gasmixture to be purified contains more than 45% of gaseous hydrogen. 3.Process according to claim 1, wherein the gas mixture to be purifiedcontains more than 70% of gaseous hydrogen.
 4. Process according toclaim 1, wherein the gas mixture additionally comprises nitrogen as animpurity.
 5. Process according to claim 4, wherein at least a portion ofthe nitrogen is adsorbed on at least one third adsorbent placed betweenthe first and second adsorbents.
 6. Process according to claim 5,wherein the third adsorbent is a zeolite.
 7. Process according to claim5, wherein the third adsorbent is a 5A zeolite.
 8. Process according toclaim 1, wherein the gas mixture additionally contains nitrogen. 9.Process according to claim 1, wherein the adsorption region ismaintained under a pressure of between 5×10⁵ and 70×10⁵ Pa.
 10. Processaccording to claim 1, wherein the adsorption region is maintained undera pressure of between 15×10⁵ and 30×10⁵ Pa.
 11. Process according toclaim 1, wherein the gas mixture to be purified is first brought intocontact with the first adsorbent selective for carbon dioxide and for C₁-C₈ hydrocarbons, and then, in a second step, with the second adsorbentof faujasite zeolite containing lithium.
 12. Process according to claim1, wherein the first adsorbent selective for carbon dioxide and for C₁-C₈ hydrocarbons is an active charcoal, a prefilter charcoal, a silicagel or a mixture thereof.
 13. Process according to claim 1, wherein thefaujasite zeolite is exchanged to at least 90% with lithium.
 14. Processaccording to claim 1, wherein the Si/Al ratio of the faujasite zeoliteis between 1 and 1.2.
 15. Process according to claim 1, wherein theSi/Al ratio of the faujasite zeolite is equal to
 1. 16. Processaccording to claim 1, wherein the ratio by weight of the first adsorbentto the second adsorbent is between 10/90 and 85/15.
 17. Processaccording to claim 1, wherein the ratio by weight of the first adsorbentto the second adsorbent is between 50/50 and 80/20.
 18. Process forcarrying out a treatment cycle comprising the stages consisting in:a)passing a hydrogen-based gas mixture contaminated by at least carbonmonoxide and containing at least one other impurity selected from thegroup consisting of carbon dioxide and C₁ -C₈ hydrocarbons into anadsorption region comprising at least: one first adsorbent bed composedof a first adsorbent selective for at least carbon dioxide and C₁ -C₈hydrocarbons and, one second adsorbent bed composed of a secondadsorbent which is a faujasite zeolite exchanged to at least 80% withlithium, and having a Si/Al ratio which is less than 1.5, for adsorbingcarbon monoxide; b) desorbing the carbon monoxide and the other impurityor impurities adsorbed on said at least first and second adsorbents bysetting up a pressure gradient and progressively lowering the pressurein said adsorption region, so as to recover the carbon monoxide and theother impurity or impurities via an inlet of said adsorption region; andc) raising the pressure in said adsorption region by introducing astream of pure hydrogen via an outlet of the adsorption region. 19.Process according to claim 18, wherein in stage a), the adsorptionregion is maintained under a pressure of between 5×10⁵ and 70×10⁵ Pa,and at a temperature of between 0 and 80° C.
 20. Process according toclaim 18, wherein in stage a), the adsorption region is maintain under apressure of 15×10⁵ and 30×10⁵ Pa, and at a temperature of between 0 and80° C.